German published patent application 199 00 445 discloses an internal combustion engine for a portable handheld work apparatus which has gas-conducting channels for fuel-rich gas and fuel-poor gas in its cylinder. The fuel-poor to fuel-free gas is supplied via outlet-near channels; whereas, the fuel component, which is necessary for the operation of the engine, is supplied via the outlet-remote channels. In this way, the fuel-free gas can screen off the outlet in the manner of an air curtain so that the fuel-rich gas does not flow out through the outlet. International published patent application WO 00/11334 discloses an internal combustion engine which has a fuel supply supported by compressed air.
The internal combustion engines, which are utilized for portable handheld work apparatus, are often configured as stratified charge engines or scavenging advance engines. The present invention is directed especially to engines of this kind.
U.S. Pat. No. 4,890,690 discloses an exhaust-gas muffler for a two-stroke engine whose housing comprises two parts which can be disassembled. A partition wall is fixedly mounted in the interior of the muffler housing and holds a catalytic converter in a through opening. The catalytic converter constitutes the flow connection for the exhaust gases between the two compartments at both sides of the partition wall.
If, in alternating sequences, exhaust gases having high oxygen concentration and oxygen-poor, hydrocarbon-rich exhaust gases from the engine reach an exhaust-gas muffler having a catalytic converter, then the oxygen-rich exhaust gases can lead to a poisoning of the active centers of the catalytic converter whereby the function of the catalytic converter is affected.
It is an object of the invention to improve the quality of the exhaust gas in an internal combustion engine of the kind described above.
The internal combustion engine of the invention includes an engine in a portable handheld work apparatus including a motor-driven chain saw, cutoff machine and blower apparatus. The internal combustion engine includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; an outlet for conducting oxygen-rich and oxygen-poor exhaust gases away from the combustion chamber; a mixture-preparation unit for supplying an air/fuel mixture; a first set of gas-supplying channels for supplying an air/fuel mixture to the combustion chamber prepared by the mixture preparation device; a second set of gas-supplying channels for supplying oxygen-rich gas to the combustion chamber; an exhaust-gas muffler having a housing and an inlet on the housing fluidly connected to the outlet to permit the oxygen-rich and the oxygen-poor exhaust gases to flow into the exhaust-gas muffler; the housing having an interior and a partition wall for partitioning the interior into a first space communicating with the outlet and a second space; a catalytic converter mounted in the housing; and, means for temporarily storing the oxygen-rich exhaust gas to thereby even out the oxygen component in the total exhaust-gas flow charging the catalytic converter.
In an internal combustion engine of the above kind, the fuel-poor to fuel-free gas is preferably supplied via outlet-near channels; whereas, the fuel component, which is necessary for the operation of the engine, is supplied via the outlet-remote channels. In this way, the fuel-free gas can screen the outlet in the manner of an air curtain so that the fuel-rich gas does not flow away via the outlet. The gas components, which flow away via the outlet, comprise substantially the fuel-poor or fuel-free but oxygen-rich gases from an early phase of the scavenging and the hydrocarbon-containing, oxygen-poor gases from a late phase of the scavenging and from the combustion. The exhaust gases get from the outlet of the engine into an inlet of an exhaust-gas muffler.
The inventors herein have determined that, in the oxygen-poor volume component of the exhaust gas, sufficient oxygen for oxidizing the hydrocarbons in the exhaust gas is no longer available which would be necessary for a high degree of conversion of the hydrocarbons. By intermediately storing oxygen from the oxygen-rich volume component of the exhaust gas, this component is utilized for oxidizing the hydrocarbons in the oxygen-poor volume component of the exhaust gas and, in this way, a higher rate of conversion is obtained.
The housing of the exhaust-gas muffler is preferably formed of two or more housing parts. A partition wall extends in the interior of the exhaust-gas muffler and partitions the housing of the muffler into at least two compartments or spaces. The partition wall is held substantially gastight with its edge at the housing of the muffler. In a preferred embodiment, a catalytic converter is held in a through opening of the partition wall and is, for example, configured as a cartridge. The housing of the catalytic converter is fixed substantially gastight in the through opening of the partition wall. The catalytic converter functions as a flow connection for exhaust gases between the inlet and the outlet of the exhaust-gas muffler.
As an alternative to the configuration of the catalytic converter as a cartridge, inner wall portions of the exhaust-gas muffler, for example, the partition wall, can be coated with a catalytically effective material.
In accordance with a first embodiment of the invention, a buffer space for oxygen-poor and oxygen-rich exhaust gas (such as for advance air and hydrocarbon-rich exhaust gas) is formed in the first compartment of the muffler housing between the inlet and the catalytic converter. A wall having a plurality of breakthroughs preferably extends through the buffer space. The wall extends in the interior of the first compartment of the muffler housing over the entire cross section thereof. In this way, a structure is provided for temporarily storing oxygen-rich exhaust gases with a storage space lying next to the inlet and a mixture space is formed for the oxygen-rich and oxygen-poor exhaust gases between the inlet of the exhaust-gas muffler and the inlet of the catalytic converter. The oxygen-rich and oxygen-poor exhaust gases, but also hydrocarbon-rich exhaust gas enter alternately one after the other into the buffer space. These gases are mixed with each other in the buffer space before they flow into the catalytic converter. The catalytic converter is protected from being charged with oxygen-rich exhaust gas. A poisoning of the active centers of the catalytic converter is thereby avoided and a permanent operation of the catalytic converter is ensured. It can be practical to provide the wall, which extends through the buffer space, with strip-shaped mutually parallel aligned breakthroughs. Preferably, an edge of the breakthroughs is bent over so that a deflection of the exhaust gases, which flow through the breakthroughs, is effected. Mixing of the exhaust gas is thereby supported.
According to a second embodiment of the invention, a means for temporarily storing oxygen-rich exhaust gases, such as the advanced air of the engine, can be provided in the region of the catalytic converter in lieu of the storage or mixture space for oxygen-rich and oxygen-poor exhaust gases arranged in the housing of the exhaust-gas muffler. For this purpose, the effective surface of the catalytic converter is provided with cerium oxides, zirconium oxides or aluminum oxides or a mixture of these oxides. These substances have an increased oxygen storage capacity whereby an oxygen poisoning of the active centers of the catalytic converter is avoided. It can furthermore be practical to mix to the above-mentioned oxides migration-retarding substances such as praseodymium or other lanthanides or actinides for stabilizing the precious metals (active centers).
It is practical to multiply deflect the exhaust-gas flow in the muffler in order to effect a limiting of the flow velocity of the exhaust gases in the interior of the exhaust-gas muffler and during the passage through the catalytic converter. Accordingly, it is preferable to mount a deflector at the inlet of the muffler on its inner side. This deflector deflects the exhaust gases flowing into the muffler. In addition, it is practical to deflect the exhaust gases between the outlet of the catalytic converter and the outlet of the muffler via a spatial offset of the outlets of the catalytic converter and the exhaust-gas muffler.
Preferably, the volume of the muffler between the inlet and the outlet is approximately 2.5 to 18 times (especially 6 to 11 times) as large as the stroke volume of the engine. This constructive measure effects an excellent mixture of the exhaust gases in the interior of the exhaust-gas muffler and a uniform passage of the exhaust gases through the exhaust-gas muffler. The volume of the catalytic converter including the oxygen store can, for example, amount to 0.3 to 10 times the stroke volume of the engine.
It can be practical to at least surround the second space of the muffler with an enclosing housing and preferably further enclose the entire exhaust-gas muffler with this housing. Cooling air is conducted in the enclosing housing which reinforces a transport of heat away from the interior of the exhaust-gas muffler. It can be practical to deflect only a component flow of the exhaust gas through the catalytic converter.